The invention is related generally to monitoring systems, and more particularly, to detecting discontinuities in a line of flowing fluid.
The infusion of fluids such as parenteral fluids into the human body is usually accomplished by means of an administration set and a metering apparatus which controls the rate of flow of the fluid through the set. Peristaltic pumps are commonly used to impact pressure to infuse the fluids as well as to meter the fluid flow.
A problem arising with the infusion of fluid into the human body is the possibility of an air embolism. While relatively small bubbles in the infused fluid may be harmless, a large enough bubble of air may result in an air embolism in the patient with fatal consequences. Dissolved gases in a liquid being infused may be released as bubbles as the liquid is subjected to pressure from a pump. Thus, monitoring for the presence of bubbles in the fluid line is a common practice where pumps are used. The monitoring is performed downstream of the pump.
Ultrasonic sensing systems are commonly used as air-in-line sensors. However, one requirement of ultrasonic systems is good ultrasonic conduction between the walls of the fluid line and the ultrasonic transducers. Continuous contact is required. A transmitter transducer is placed opposite the fluid line from the receiver transducer and energy is transmitted between the two through the fluid line. Air has a substantially different acoustic impedance in comparison to liquid and air in a liquid flowing in the fluid line between the two transducers will be detected by the substantial change in the amount of ultrasonic energy received by the receiver. However, the ultrasonic sensor cannot distinguish between an air gap caused by poor contact between the transducer and the fluid line and an air bubble in the fluid flowing in the fluid line. Thus, contact directly affects sensor performance.
With one piece ultrasonic sensor housings, ultrasonic energy tends to find its way through the sensor housing from one transducer to the other without passing through the fluid line. This results in an undersirable background noise level which reduces the sensitivity of the sensor system. Accordingly, a two sensor housing arrangement is preferred. Each sensor housing has a transducer and the two sensor housings have an air gap between them when they are engaged with each other which acts to block the transmission of ultrasonic energy from reaching the receiver transducer unless it proceeds through the fluid line being monitored.
A design goal of two-housing arrangements for ultrasonic sensors in the medical field is ease of use. Administration sets are typically disposable and the ultrasonic sensor unit should be able to accept numerous sets. Correct placement of the fluid line between the two housing parts should be easy and quick to accomplish. Where the sensor is co-mounted with a peristaltic pump, ease of tubing engagement with the air-in-line sensor after pump engagement is desirable. Means to hold the fluid line in position once engaged should be provided so that operation of an associated pump or patient movement do not cause the fluid line to dislodge from the sensor and provide a false alarm. Additionally, the two-part housing should have the ability to compensate for tubing dimensional tolerances and size changes due to differing manufacturing tolerances, temperature and pressure variations and other factors which may affect the tubing. A simple engagement system is preferred but also one which will result in the tubing being firmly engaged by the transducers so that good transducer/tubing contact exists with the result of reduced noise levels. However, the engagement of the tubing with the transducers should not be to such an extent that fluid flow is excessively restricted.
Hence those concerned with fluid line monitoring have recognized that it would be benefical to provide a versatile fluid monitoring system and method useful for detecting air in the line, which can be used with disposable administration sets under varying conditions, yet is accurate and does not excessively restrict fluid flow. The present invention fulfills these needs.